Rectal Cancer: Preoperative Evaluation and Staging

Marcet, Jorge

doi:10.1007/978-3-030-01165-9_27

Jorge Marcet⁷

2813 Accesses

Abstract

Preoperative staging of the patient with rectal cancer can provide valuable information to guide the selection of treatment options. The modalities used for preoperative staging include CT, ERUS, MR, and PET, each with its own advantages and disadvantages. These tests are complimentary, and staging accuracy improves when the tests are used in combination. The resolution of these tests drops in detection of micrometastases, and thus patients may be understaged. In contrast, patients with a significant peritumoral tissue reaction may be overstaged. Treating physicians should remain cognizant of the possibility of understaging or overstaging patients when making therapeutic plans.

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The Preoperative Staging of Rectal Cancer

Rectal Conditions: Rectal Cancer—Adjuvant and Neoadjuvant Therapy

Colorectal Cancer: Preoperative Evaluation and Staging

Keywords

FormalPara Key Concepts

Accurate preoperative staging of patients with rectal cancer helps identify patients at risk for local or distant metastasis and guides treatment decisions.
Endorectal ultrasound (ERUS) is effective for staging the depth of invasion (T stage), especially for early-stage rectal tumors (uT0, uT1) that may be considered for local excision.
Magnetic resonance (MR) has the ability to delineate the extent of locally advanced tumors and estimate involvement of the mesorectal fascia.
ERUS and MR use surrogate markers to estimate nodal involvement—size, node morphology—and are not particularly accurate in predicting nodal metastatic spread unless there are multiple large nodes in the mesorectum.
The potential for understaging and overstaging of patients should be realized and taken into account when making treatment decisions.
High-resolution computed tomography (CT) can detect distant metastatic lesions greater than 1 cm in diameter.
Positron emission tomography (PET) scan is the most accurate assessment of total body tumor burden, especially when combined with CT (PET-CT).
PET-CT is indicated when there are equivocal findings on CT, and finding distant metastatic disease would alter therapeutic decisions.

Introduction

Preoperative staging is performed according to the TNM classification of malignant tumors, estimating the depth of invasion into the rectal wall (cT), the presence or absence of lymph node metastasis (cN), and presence of distant metastasis (cM). Also of importance is the determination of invasion of the anal sphincter and pelvic floor musculature, adjacent pelvic organs, or pelvic sidewall, all with significant consequences of planning and treatment to the patient.
The prefix “c” is used to indicate clinical staging, which is the estimate of stage based on physical examination and radiographic studies. Unfortunately, there is often confusion regarding this distinction, with some authors describing treatment recommendations for “T3N0” tumors as determined by pretreatment staging, when instead they should describe the tumor as “cT3N0.” The difference at first glance appears trivial but can have significant consequences if the clinician fails to understand that estimates of tumor stage are just that, estimates, and that treatment planning must take into account the potential inaccuracy of these estimates. For example, understaging of the cancer preoperatively may result in the omission of preoperative radiotherapy/chemoradiotherapy and lead to an increased risk of local recurrence. Conversely, overstaging may lead to overtreatment, increasing the overall morbidity and cost of treatment.
Pretreatment evaluation begins with physical examination and colonoscopic evaluation. Radiographic studies may include computed tomography (CT), endorectal ultrasound (ERUS), magnetic resonance imaging (MRI), and positron emission tomography (PET). These tests are complimentary, each with their own advantages and disadvantages, and may be used in combination. Laboratory evaluation includes determination of the carcinoembryologic antigen (CEA) level.

History and Physical Examination

When evaluating a patient diagnosed with rectal cancer, the patient’s history is recorded, and an inquiry is made as to the duration of symptoms, changes in weight, bowel habits, bowel control, and presence of pain.
If restorative proctectomy or local excision is to be contemplated, a detailed assessment of anal sphincter function and prior trauma (e.g., obstetrical history, prior anal operations) should be obtained.
A general physical examination is performed with special attention for signs of muscle wasting, abdominal distension, hepatomegaly, and lymphadenopathy.
A careful digital rectal examination is performed, noting the distance of the tumor from the anal verge and its proximity to the anal sphincter and pelvic floor. Tumors located in the anterior portion of the rectum have the risk of invasion into the genital structures, and special attention should be made to the potential for fixation to adjacent structures (i.e., prostate, vagina, sacrum, puborectalis). In a woman with an anterior rectal cancer, a pelvic examination should be done to ensure there is no invasion of the vaginal wall that may affect treatment. When the tumor is located in the posterior or lateral rectal wall, pelvic sidewall invasion should be considered.
If restorative proctectomy is being considered, assessment of anal sphincter bulk and tone is important as it may help predict postoperative function.
The texture of the tumor also gives a clue as to the stage. Benign adenomas are soft, and the tumor may occasionally be difficult to detect on digital rectal examination. When a tumor invades the rectal wall, a desmoplastic reaction occurs, and the resulting fibrosis will be felt as firm tissue.
Evaluating the mobility of the tumor can also give information on how deep the tumor invades. A tumor tethered to the rectal wall, but that is otherwise mobile, is likely to invade into but not through the wall. Tumors that are fixed within the pelvis and are not mobile are locally advanced, deeply invading the full thickness of the rectal wall and possibly invading surrounding pelvic structures.
The digital rectal examination may occasionally also detect peritumoral lymphadenopathy , though this is often difficult. It should be noted that digital rectal examination has limitations in that only tumors of the distal rectal rectum can be adequately assessed. Furthermore, accuracy in staging depth of invasion is better for advanced tumors than for early tumors and improves with the surgeon’s experience.

Endoscopic Evaluation of the Rectum

Flexible sigmoidoscopy or proctoscopy should be performed to help localize the tumor anatomically and assess its appearance.
The endoscopic appearance of a tumor also gives a clue as to the relative degree of invasion, with benign tumors soft to manipulation with the endoscope or endoscopic forceps and malignant tumors being firm. Ulceration of the tumor implies invasion into the rectal wall, while deep ulceration may be a sign of transmural invasion.
Distance to the anal verge is best assessed by rigid proctoscopy, although this measurement is of limited utility as it can vary greatly based on differences in body habitus. It is more important to assess the distance of the distal margin of the tumor from the anorectal muscular ring as this will often guide the decision between restorative and non-restorative proctectomy. Another assessment which is helpful is the relationship of the tumor to the folds of Houston.
As noted in other chapters, the surgeon should always examine the rectum of any patient referred with a lesion in the left colon prior to operation, as flexible endoscopic measurements of distance by non-surgeons are notoriously inaccurate. Many lesions described as being proximal to “15 cm” are actually in the true rectum. This discovery may fundamentally alter treatment planning.

Total Colon Evaluation

Evaluation of the proximal colon , preferably by complete colonoscopy, should be performed in all patients with rectal cancer to exclude synchronous lesions and to confirm the histopathology of the tumor via biopsy.
Other radiological testing may occasionally be used (i.e., CT colonography, air-contrast enema) for patients who cannot undergo complete colonoscopy, though each has inherent limitations that providers should be aware of such as the need for an adequate preparation or failure to identify small lesions.
Patients that are unable to be cleared prior to surgery due to an obstructing lesion should undergo proximal colon evaluation within 6 months after their operation.
In select cases of an apparent benign lesion, pretreatment evaluation may be limited to digital rectal examination, colonoscopy, and CEA prior to surgery.
For patients with known or suspected rectal invasive adenocarcinoma, additional pretreatment staging is appropriate.

Locoregional Imaging

Computed Tomography

Although computed tomography (CT) is routinely performed to exclude distant metastatic disease, it has limited ability to define the mesorectal fascial layers and layers of the rectal wall. Although CT can suggest tumor invasion into surrounding structures, tumor involvement of an adjacent organ or the pelvic sidewall is not entirely accurate and is only inferred by the loss of the fat plane between the tumor and the adjacent organ or structure.

Endorectal Ultrasound

On endorectal ultrasound (ERUS) , the bowel wall is defined by five distinct sonographic layers of alternating hyper- and hypoechoic qualities. Extending from the lumen outward, these layers correspond to (1) the interface between the ultrasound probe and the mucosa, (2) the interface between the mucosa and muscularis mucosa, (3) the submucosa, (4) the muscularis propria, and (5) the serosa or pericolic fat. The prefix “u” is used to describe ERUS, T, and N staging of rectal cancer (Figs. 27.1, 27.2, 27.3, 27.4 and 27.5).
The advantage of ERUS is that it can be performed in the surgeon’s office as part of the initial evaluation of the patient, and it is inexpensive compared to CT or MR. The patient is given an enema to evacuate the rectum prior to the procedure. The procedure is often combined with a flexible or rigid proctosigmoidoscopy. The probe can be passed through a rigid proctoscope to assess proximal tumors.
The ultrasound probe needs to pass proximal to the tumor in order to evaluate the entire extent of the tumor, thus making it difficult or impossible with obstructing lesions.
3-D ultrasonography records the image in real time and allows for subsequent manipulation of the image for axial, coronal, and sagittal evaluation.
Malignant lymph nodes appear as hypoechoic and rounded peritumoral structures, whereas benign lymph nodes are less likely to be detected as they are isoechoic with the perirectal fat.
Limitations to ERUS for staging rectal cancer include incomplete exams due to tumors that are bulky or stenotic and inadequate contact of the ultrasound probe with the tumor due to air or stool in the rectum or angulation of the tumor. Operator experience has also been shown to play a role in the accuracy of ERUS staging. Some patients require sedation to allay discomfort or anxiety.

T Staging

The reported accuracy of ERUS in accessing the T stage of rectal cancer ranges from 63% to 96% (Table 27.1).
As with many interpretive studies, operator experience plays a significant role in staging accuracy.
Several investigators have demonstrated a lower accuracy of ERUS in detecting T2 tumors compared to T1, T3, or T4.
However, other investigators have demonstrated the utility of ERUS in the selection of patients with early-stage rectal cancer (T0, T1) who may benefit from transanal excision instead of traditional transabdominal rectal resection.

Table 27.1 ERUS accuracy compared to histological stage

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N Staging

Accuracy for detecting metastatic lymph nodes by endorectal ultrasound is less precise than for T staging , with a variable accuracy in reported studies of 63%–85%.
Further complicating the analysis is that different investigators have used different size and morphology criteria for nodal involvement with tumor.
Staging accuracy for lymph node metastasis improves when the findings are associated with the T stage, with a higher risk of metastasis correlating with higher T stage.

Magnetic Resonance

High-resolution magnetic resonance (MR) with phased array pelvic coils is being increasingly used in the preoperative assessment of rectal cancer given its improved ability to evaluate the at-risk surgical circumferential resection margin (Table 27.2).
The pelvic coil is a wraparound surface coil placed around the pelvis. Patients are prepared with an enema on the morning of the examination. Thin-section (3-mm) T2-weighted fast spin-echo sequences are obtained in a plane orthogonal to the tumor. Higher-resolution MRI allows improved definition of bowel and tumor infiltration.
MR with endorectal coil is no longer recommended. Although endorectal MRI can show five layers of the rectal wall, the field of view is limited, and the mesorectal fascia is not always visible. Additionally, the endorectal coil is more uncomfortable to the patient than the external coil and cannot be inserted in stenosing tumors. Endorectal coil also has the potential to distort the tissues.
Three layers of the rectal are visible on a phased array external MR. The innermost mucosa is thin and hypointense, the middle submucosa is hyperintense, and the outer muscularis propria is darkly hypointense.
Below the peritoneal reflection, the rectum is surrounded by the mesorectal fat (MRF) which is limited by the thin mesorectal fascia, which fuses with the retroprostatic or retrovaginal fascia anteriorly and the presacral fascia posteriorly. The MRF surrounds the rectum completely only in the lower third and is best seen laterally as a thin hypointense line on T2W sequences. Inferiorly, the MRF thins out as it reaches the levator ani, which forms the roof of the ischiorectal fossa.
MR is the best imaging modality to identify this avascular plane surrounding the mesorectum, which includes the mesorectum in its fascial envelope—the circumferential radial margin (CRM) (Fig. 27.6) and invasion of the anal sphincter musculature (Fig. 27.7).
As with other radiographic techniques, prediction of N stage is less accurate than for T stage. However, MR appears to be the most accurate of the imaging modalities currently employed. A variety of advanced techniques are being employed by various investigators in an attempt to improve nodal staging accuracy with MR.
MR limitations include foreign bodies in patients that are MR incompatible. Foreign bodies that are compatible, such as surgical clips, may also obscure images. Movement-related artifacts may preclude accurate visualization of the rectal wall. MR is not portable to the operating room and is more expensive than ERUS.
Many referral centers with an expertise in rectal cancer treatment are now utilizing MR as the preferred locoregional staging evaluation, especially for locally advanced tumors. ERUS is utilized for evaluation of early-stage lesions or used in combination with MR for select patients.

Table 27.2 Meta-analysis of magnetic resonance accuracy in T stage, N stage, and circumferential resection margin (CRM)

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